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Wed, 17 Apr 2024 14:47:32 -0700 (PDT) MIME-Version: 1.0 References: <20240417212549.2766883-1-peterx@redhat.com> In-Reply-To: <20240417212549.2766883-1-peterx@redhat.com> From: Pasha Tatashin Date: Wed, 17 Apr 2024 17:46:56 -0400 Message-ID: Subject: Re: [PATCH v4] mm/page_table_check: Support userfault wr-protect entries To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Nadav Amit , Andrew Morton , Axel Rasmussen , David Hildenbrand Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: qm9z5z6gitphmzpk1immpeemxf75cx8r X-Rspamd-Queue-Id: 92E33120003 X-Rspamd-Server: rspam06 X-Rspam-User: X-HE-Tag: 1713390453-269027 X-HE-Meta: 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 ycxKiSC4 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Apr 17, 2024 at 5:25=E2=80=AFPM Peter Xu wrote: > > Allow page_table_check hooks to check over userfaultfd wr-protect criteri= a > upon pgtable updates. The rule is no co-existance allowed for any writab= le > flag against userfault wr-protect flag. > > This should be better than c2da319c2e, where we used to only sanitize suc= h > issues during a pgtable walk, but when hitting such issue we don't have a > good chance to know where does that writable bit came from [1], so that > even the pgtable walk exposes a kernel bug (which is still helpful on > triaging) but not easy to track and debug. > > Now we switch to track the source. It's much easier too with the recent > introduction of page table check. > > There are some limitations with using the page table check here for > userfaultfd wr-protect purpose: > > - It is only enabled with explicit enablement of page table check confi= gs > and/or boot parameters, but should be good enough to track at least > syzbot issues, as syzbot should enable PAGE_TABLE_CHECK[_ENFORCED] for > x86 [1]. We used to have DEBUG_VM but it's now off for most distros, > while distros also normally not enable PAGE_TABLE_CHECK[_ENFORCED], whi= ch > is similar. > > - It conditionally works with the ptep_modify_prot API. It will be > bypassed when e.g. XEN PV is enabled, however still work for most of th= e > rest scenarios, which should be the common cases so should be good > enough. > > - Hugetlb check is a bit hairy, as the page table check cannot identify > hugetlb pte or normal pte via trapping at set_pte_at(), because of the > current design where hugetlb maps every layers to pte_t... For example, > the default set_huge_pte_at() can invoke set_pte_at() directly and lose > the hugetlb context, treating it the same as a normal pte_t. So far it'= s > fine because we have huge_pte_uffd_wp() always equals to pte_uffd_wp() = as > long as supported (x86 only). It'll be a bigger problem when we'll > define _PAGE_UFFD_WP differently at various pgtable levels, because the= n > one huge_pte_uffd_wp() per-arch will stop making sense first.. as of no= w > we can leave this for later too. > > This patch also removes commit c2da319c2e altogether, as we have somethin= g > better now. > > [1] https://lore.kernel.org/all/000000000000dce0530615c89210@google.com/ > > Cc: Pasha Tatashin > Signed-off-by: Peter Xu > --- > v2: > - Rename __page_table_check_pxx() to page_table_check_pxx_flags(), > meanwhile move the pte check out of the loop [Pasha] > - Fix build issues reported from the bot, also added SWP_DEVICE_WRITE whi= ch > was overlooked before > v3: > - Add missing doc update [Pasha] > v4: > - Fix wordings in doc, use more elegant swap helpers [Pasha] > --- > Documentation/mm/page_table_check.rst | 9 +++++++- > arch/x86/include/asm/pgtable.h | 18 +--------------- > mm/page_table_check.c | 30 +++++++++++++++++++++++++++ > 3 files changed, 39 insertions(+), 18 deletions(-) Reviewed-by: Pasha Tatashin Thanks, Pasha